1. Field of the Invention
The present invention relates to a processing system with a galvano scanner which is capable of high speed laser scanning of a workpiece.
2. Description of the Related Art
In general, a “galvano scanner” is a device which comprises two mirrors which can rotate about two mutually perpendicular rotational axes, and is configured to change scan path of the laser beam on a workpiece by driving rotation of these mirrors using servo motors (galvano motors). Galvano scanners are broadly used in applications for marking workpieces with bar codes or manufacturing serial numbers etc. at a high speed. Galvano scanners are also used for the recently increasingly popular laser sintering type 3D printers and other such stereolithographic apparatuses. Such a stereolithographic apparatus produces a desired 3D shape model by emitting a laser beam to metal powder or a photocurable resin etc. formed into a thin layer so as to sinter or cure the metal powder.
FIG. 11 and FIG. 12 are schematic views which show the routine of a laser sintering operation in a general stereolithographic apparatus in a time series manner. The two figures are plan views of metal powder formed in a layer on a table as seen from above. The galvano scanner first makes the laser beam scan the surface along a path which is shown by the arrows A11 in FIG. 11 so as to sinter the metal powder M. After that, a new thin layer of metal powder M is supplied, and the galvano scanner then makes the laser beam scan the surface along the path which is shown by the arrows A12 in FIG. 11 so as to sinter the metal powder M. In this way, a general laser sintering operation involves a change in the scan direction of the laser beam for sintering the metal powder M each time a new thin layer of metal powder M is supplied. Such a sintering operation of each layer is repeatedly performed to produce a desired 3D shape model.
As will be understood from FIG. 11 and FIG. 12, in the sintering operation of each layer of the metal powder M, the top surface of the model under processing is divided into a plurality of belt-shaped regions R which have predetermined widths (for example, 5 mm). Each of these belt-shaped regions R is scanned by the laser beam. Further, in each of the plurality of belt-shaped regions R, the laser beam repeats back and forth motion at a high speed along the traverse direction of the belt-shaped region R so as to sinter the metal powder M (see arrows A11 in FIG. 11 and arrows A12 in FIG. 12).
In order to sinter a thin layer of metal powder uniformly without irregularities, the laser intensity and laser scan speed are preferably kept constant while the laser beam is moved back and forth. The laser beam is stopped for an instant when the back and forth motion of the laser beam is reversed, the laser control may be designed to turn the laser output off at the time of the reversal of the laser beam so as to prevent uneven sintering (i.e., excessive sintering) due to the reversal. FIG. 13 is a schematic view for explaining the above laser output control. More specifically, FIG. 13 schematically shows the scan path of a laser beam in a general laser sintering operation. In the scan path of the laser beam which is shown by the reference notation P in the figure, the output of the laser beam is temporarily turned off during the periods when the laser beam is stopped. These periods are shown by the broken lines.
FIG. 14 is a graph which shows a temporal change of a scan speed of laser beam in a general laser sintering operation. More specifically, the graph of FIG. 14 is a graph which shows the temporal change of the scan speed of a laser beam while the laser beam makes one back and forth motion in a belt-shaped region R in FIG. 11 or FIG. 12. As shown in FIG. 14, the laser beam is accelerated, moved at a constant speed, and decelerated in the forward direction of the servo motor before being temporarily stopped. Then the laser beam is accelerated, moved at a constant speed, and decelerated in the reverse direction of the servo motor before being stopped again. The drive commands (acceleration/deceleration commands) to the galvano scanner for these periods has stepped forms, but there is always a delay due to the response time of the galvano scanner, and therefore the actual laser beam is accelerated or decelerated with a certain time constant, as shown in FIG. 14. That is, in a conventional galvano scanner, responsiveness of a drive command is limited due to the acceleration ability of the servo motor, and therefore it is difficult to accelerate the scan speed while maintaining the scan precision of the laser beam.
In general, the time constant of on/off control of the laser output is far smaller than the time constant of acceleration/deceleration control of a galvano scanner, and therefore uneven sintering tends to occur in the acceleration/deceleration phase of a galvano scanner. In relation to this, JP2008-170579A proposes a control method which generates a laser output waveform corresponding to the drive command of the galvano scanner. More specifically, the control method of JP2008-170579A delays the timing of emitting the laser beam in accordance with a time constant of the acceleration/deceleration command of the galvano scanner, in an attempt to reduce the uneven sintering. However, the actual operation of the galvano scanner is delayed somewhat from the drive command, and therefore uneven sintering cannot be completely eliminated even if a laser output waveform is generated corresponding to the drive command of the galvano scanner.
A processing system which can accelerate the scan speed of a laser beam by a galvano scanner while maintaining the scan precision is being sought.